The growing demand for materials possessing nonlinear optical properties for uses ranging from optical communications to optical storage and retrieval has spurred the search for materials ever since the discovery of nonlinear optic phenomena several decades ago. A useful description of the development of nonlinear optic materials was written by S. Allen in New Scientist, pp. 59-63, July 1, 1989. It is pointed out therein that while several crystalline inorganic compounds are known to possess sufficient nonlinearity for a variety of uses. Organic compounds, which often have much higher nonlinearities, hold the potential for more efficient use. Furthermore, nonlinearity can be designed into a given organic molecule by selecting an effective combination of electron donating and electron accepting groups, as taught, for example, by Ulman et al., U.S. Pat. No. 4,792,208. Such nonlinear organic molecules, when dispersed in polymeric binders, can have the additional advantages of ease of manufacture into a wide variety of shapes and low susceptibility to damage in use.
Second order nonlinear optical phenomena such as SHG (second harmonic generation), sum and difference frequency generation, parametric processes and electro-optical effects, all arise from the second order polarization susceptibility .chi..sup.(2). For significant nonlinear optical phenomena it is desirable that a molecule possess a large hyperpolarizability, .beta., and that the macroscopic form of the molecule, that is, the ensemble of such molecules, possess a large .chi..sup.(2).
Media that have a polarization susceptibility have been recognized as providing manipulating beams of incident electromagnetic radiation. Such media are said to possess nonlinear polarization. The effects attributable to such nonlinear polarization are a property of the medium. To obtain the highest nonlinear polarization property, the molecules within the medium must be arranged so that the nonlinear properties of the individual polar molecules within the medium do not cancel each other out.
Polar molecules may exist in the form of charge-transfer complexes. Charge transfer complexes generally are known in the art. See "Organic Charge-Transfer Complexes", R. Foster, Academic Press, New York, 1969, and A. Weller, "Exciplex" edited by M. Gordon; W. R. Ware, Academic Press, N.Y., 1975. A charge-transfer complex, as known in the art, is formed by interaction of two or more component molecules which are in reversible equilibrium with its components. No covalent bonding exists between the components. Charge transfer complexes are bound together by the partial donation of electrons from at least one component molecule to at least one other component molecule.
On a molecular level the polarization of a nonlinear optical material can be described by the expression: EQU .mu.=.mu..sub.0 +.alpha.E+.beta.EE+.gamma.EEE+. . .
where:
.mu. is the induced dipole moment of the molecule; PA1 .mu..sub.0 is the permanent dipole moment of the molecule; and PA1 E is an applied electric field. PA1 P is the induced polarization in the ensemble of molecules; PA1 P.sub.o is the permanent polarization in the ensemble of molecules; and PA1 E is the applied electric field.
Coefficients .alpha., .beta., and .gamma. are tensors that represent linear, second order and third order polarizabilities, respectively. First order or linear polarization is described by .alpha.E; second order or first nonlinear polarization by .beta.EE; and third order or second nonlinear polarization by .gamma.EEE.
The polarization of an ensemble of molecules induced by an applied electric field is described by the expression: EQU P=P.sub.0 +.chi..sup.(1) E+.chi..sup.(2) EE+.chi..sup.(3) EEE+. . .
where:
Coefficients .chi..sup.(1), .chi..sup.(2) and .chi..sup.(3) are tensors that represent linear, second order and third order polarization susceptibilities, respectively.
.chi..sup.(2) arises from the second order molecular polarizability or first hyperpolarizability, .beta., and .chi..sup.(3) arises from further higher order hyperpolarizabilities. As tensor quantities, the susceptibilities, .chi..sup.(i), depend on the symmetry of the molecular ensemble; odd order susceptibilities are nonvanishing for all materials, whereas even order susceptibilities such as .chi..sup.(2), are nonvanishing only for noncentrosymmetric materials.
Franken et al., Physical Review Letters, 7, 118-119 (1961), disclose the observation of second harmonic generation (SHG) upon the projection of a pulsed ruby laser beam through crystalline quartz. The use of a laser remains the only practical way to generate an E large enough to be able to detect the SHG phenomenon.
Although organic molecules are known to possess optical non-linearity, the need exists for organic molecules which exhibit exceptionally high second order molecular polarizabilities relative to conventional nonlinear optically active organic materials. Such materials, in accordance with the invention, can be made into nonlinear optic elements useful in a wide variety of applications requiring high nonlinearity.